Freshwater shortage is an escalating global problem, which is being exacerbated by global climate change, particularly the ever-expanding arid equatorial region, and by pollution of freshwater sources in rapidly industrializing emerging economies.
Existing methods of water treatment and desalination tend to have high costs associated with the infrastructure required to construct and maintain water treatment plants and with the energy required to run them. One approach to solving this problem has been the development of humidification-dehumidification (HDH) systems, in which water is evaporated and then re-condensed. HDH systems can be designed to use solar energy, an environmentally friendly and inexpensive energy source that is abundant in hot and arid areas. HDH desalination uses separate components for each of the thermodynamic processes in water purification, thereby allowing each component to be designed independently and allowing much greater flexibility in the design of the thermodynamic cycle used for evaporating water and then condensing the water vapor back into liquid water.
Japan Pat. Appl. Pub. No. 61-291093 discloses an apparatus for desalination of sea water. The apparatus comprises a container in which, when irradiated by solar radiation through a transparent plate, solar heat is efficiently absorbed by a thin layer of sea water on a heat absorbing heat insulating film. The temperature of the sea water at that part is effectively raised, generating steam having a pressure corresponding to the saturated steam pressure at that temperature. A fan draws a mixture of this steam and air into a suction pipe and from there into a condensing pipe group. Because the condensing pipe group is sufficiently low in temperature compared with the surface of the sea water layer present above the film, part of steam introduced into the condensing pipe group is condensed, generating fresh water which is, in turn, sent to a water storage tank.
U.S. Pat. No. 4,172,767 discloses a water purification system that is placed within a source of impure water. The system comprises a container for holding impure water, an upper portion of which is above the surface of the source of impure water; a cover of flexible plastic for the container, which causes heat from solar radiation impinging thereon to accumulate in the container, thus vaporizing water within; cloth covered baffles in the upper portion of the container for increasing the evaporation area; a blower for moving vapor across the upper portion of the container; a vapor pipe for receiving and transporting vapor from an area of one natural temperature through an area of a lower natural temperature, thereby condensing the vapor; and a pipe and a pump for transporting the condensed water to a storage tank.
U.S. Pat. No. 4,187,151 discloses a desalination device. The device comprises a base with a transparent or translucent chamber that floats on the surface of a body of salt water. Waves cause the chamber to expand and contract, and valves cause air to be pumped through the chamber. The moisture-laden air leaving the chamber is led to a condensation and collection tank below the level of the water.
U.S. Pat. No. 4,219,387 discloses a still a solar still adapted to float on a body of water. The still comprises a toroidal evaporating chamber with sunlight admitting and absorbing, respectively, top and bottom walls for vaporizing water from the body admitted to overlie the bottom wall. A surrounding inner float ring and underlying toroidal inflatable float support the chamber. A condenser depends from and communicates with the evaporating chamber through elongate coaxial vapor outlet and air return tubes, and in turn supplies distillate to a pendent holding tank. A rotatable shaft extending coaxially down through the evaporating chamber carries a fan to propel vapor from the evaporating chamber into the condenser due to rotation of a windmill atop the chamber. A curved reflector is rotatably driven atop the inner ring to direct additional sunlight on the evaporating chamber as the sun moves overhead. An outer float ring loosely coaxially surrounds the inner float ring. The annular water surface between the float rings, covered by a transparent film, forms an oxygen production zone occupiable by oxygen producing phytoplankton fed by nutrients in water brought up from beneath the thermocline by thermosiphon flow between the warm condenser and a surrounding heat skirt. Pump units mounted on the outer float ring remove distilled water and any oxygen produced, the latter for example to a device for dissolving the oxygen below the thermocline in the body of water.
U.S. Pat. No. 6,494,995 discloses a desalination device, A solar cell floats over a body of saline water. A submerged fresh water collection system underlies the cell. A partial vacuum is created in the solar cell for drawing water vapor from the cell to the collection system. Water vapor is condensed in a condenser disposed between the cell and the collection system. Heat generated by the condensation of water vapor is utilized to heat the salt water, which rises upwardly to replace the salt water vaporized in the cell.
Spanish Pat. No. 2110914 discloses a system for desalinating sea water and generating electrical energy using solar energy as heat source. A reflector assembly is formed by parabolic mirrors captures the solar energy in the boiler, where the water is converted into steam which, either directly or after converting the pressure thereof into electrical energy in a turbogenerator, is converted in a chiller into drinking water which accumulates in a tank. The mirror/boiler/chiller assembly is on a rotary platform above the pool which serves, at one and the same time, as reservoir, boiler base and chiller.
U.S. Pat. Appl. Pub. No. 2014/0197022 discloses a solar-powered HDH desalination system that includes a supply of saline or brackish water that passes through a dehumidifier/condenser. The saline or brackish water is preheated in the dehumidifier/condenser as a result of its condensation. A plurality of humidifying stages includes humidifiers and associated solar collectors. The solar collectors heat air, and the heated air then passes through a series of humidifiers to evaporate the preheated saline or brackish water, thereby separating pure water from the brine. The humid air is reheated and recirculated through the humidifying stages and the dehumidifier, and the desalinated water from the dehumidifier via condensation is collected and processed. The system recirculates the brine successively from each humidifier to the next for more efficient evaporation of the water and to lower energy consumption.
It can be seen from the preceding discussion that the systems known in the art lack important desirable features such as portability, modularity, low construction and maintenance costs, and high efficiency to allow energy-efficient production of fresh water from sources such as salt water, brackish water, or black water, at a reasonable cost and without a requirement that the water purification unit be physically located within the source of contaminated water. Thus, development of a system that meets these requirements remains a long-felt but as yet unmet need.